Trees (1992) 6:225-231 9 Springer-Verlag1992 Whole-plant CO 2 exchange of seedlings of two Pinus sylvestris L. provenances grown under simulated photoperiodic conditions of 50 ~ and 60~ J. Oleksyn 1, M. G. Tjoelker 2, and P. B. Reich 2 l Polish Academy of Sciences,Institute of Dendrology,Parkowa5, PL-62-035Kornik,Poland 2 Universityof Minnesota, Departmentof Forest Resources, St. Paul, MN 55108, USA ReceivedNovember5, 1991/Accepted April 16, 1992 Summary. Seedlings of Scots pine (Pinus sylvestris L.) from Russia (59~ and Poland (53~ were grown for 4 months in controlled environment chambers, simulat- ing the photoperiod conditions of 50 ~ and 60~ The Russian population grown at 50~ showed earlier height growth cessation than the Polish population. Photoperiodic conditions of 60~ increased proportional allocation of dry mass to shoots and lowered allocation to roots in the Russian population, which also had greater allocation to roots than the Polish population in both treatments. Total non-structural carbohydrate concentrations in roots and secondary needles of both populations were significantly higher at the end of the 4 month growing season at 50 ~ compared to 60 ~ N. Net photosynthesis rates were similar for both provenances and both treatments. The rate of transpiration was higher and water-use efficiency lower for plants grown in long-day conditions of 60 ~ N. The mean respiration rate of roots ranged between 30 and 36 nmol CO2 9 g-1 dry mass - s-1 and was 2-4 times higher than values observed for needles. Root respiration rates were greater in the Polish than the Russian population. Despite this, the greater allocation to root dry mass of the Russian population resulted in greater root respiratory cost as a proportion of daily carbon gain. Overall, root respiration accounted for between 18 to 34% of the total daily net carbon assimilation of these populations. Root and total respiration as a proportion of net daily carbon assimilation were greater at 50" than 60~ Mean net integrated CO2 gains were 2.2-2.5 mmol CO2 9 day -1 for seedlings from Russia compared to 3 mmol CO2 - day -1 for Poland. Key words: Scots pine - Photoperiod - Growth cessation - Water-use efficiency - Carbon gain Correspondenceto: P. B. Reich Introduction The dependence of Scots pine (Pinus sylvestris) seedling growth on photoperiod was known from the early 1930s (B ogdanov 1931) and later (Wareing 1950 a, b 1951; Was- sink and Wiersma 1955; Downs and Borthwick 1956; Vaartaja 1959; Koski and Siev~inen 1984). The influence of photoperiod can be especially important for plants grown from seeds transferred long distances from the place of origin. Results of provenance experiments with Scots pine showed a significant differentiation in growth among populations with respect to long distance transfer of plants. It was found that northern populations, from 57- 67~ N, are sensitive to transfer to the south, as evident in reduced height growth and above ground volume production com- pared to their native latitude (Giertych 1979; Giertych and Oleksyn 1981). At the same time, populations originating from the central part of the species range in Europe, from 54-57 ~ N, grow as well or better at the northern latitudes as local provenances. The nature of these growth differences is not fully understood. Most of the controlled experiments in this field have been performed with fixed day/night photoperiod and tem- perature regimes, which deviate far from the natural varia- tion of daylength and temperature at any given latitude. Under such conditions it is not possible to judge whether growth or other changes are due to photoperiod or are a result of differences in temperature or quantum flux den- sity over the experimental period or other factors (Oden and Dunberg 1984). In contrast to previous studies our treatments tracked the natural changes in day and night length over the course of the growing season. Also, diurnal variation in incident light was introduced to simulate light changes over the course of the day. At the same time daily integrated light levels were balanced among the treatments in order to eliminate the possible confounding effect of unequal quantum flux due to differences in daylengths. Relatively little is known concerning photosynthesis, respiration and/or whole plant carbon balance of woody plants under different photoperiods. The weakest link in such estimations is, in general, insufficient information